In addition to closed captioning information, television signals in the United States may also include Extended Data Services (EDS or XDS) information. EDS information will be available on line 21 of field 2 and includes data encoded in the same format as closed caption data. Thus, the video signal includes both a video information signal component and a data signal component comprising EDS and caption data. EDS and captioning share the bandwidth of line 21 field 2 in a time multiplexed manner. EDS offers a wide range of useful information such as: program title, network name, and current time of day.
The format of EDS data is specified in the EIA-608 standard for line 21 data services for NTSC television signals. EDS data is organized in packets of data bytes. Each packet conveys one piece of information, e.g., the current time of day. Each line 21 of field 2 provides two bytes of EDS data. Particular data byte values occurring in the first byte of any line 21 of field 2 indicate the start of a packet. Each "start" data byte also defines a "class" of information that is included in the packet. For example, a "current" class packet includes information pertaining to the current program that is being viewed. A "miscellaneous" class packet includes various types of information such as time of day information. As an example, a value of 01h in the first data byte in line 21 of field 2 indicates the start of a "current" class packet while a value of 07h indicates the start of a "miscellaneous" class packet.
Although EDS information is potentially useful, EDS data decoded in a receiver may provide incorrect or inappropriate information. EDS data, like closed captioning data, occurs at a data rate that permits recording EDS data on video tape via a conventional video cassette recorder (VCR). Thus, recording a television signal that includes EDS information will also record the EDS data, e.g., EDS time-of-day packets that provide the current time when the packet was recorded. When a tape is viewed at a later time, recorded EDS information can be detected and decoded 0 by the receiver. If the receiver always uses received EDS information for control purposes, the receiver may respond in an undesirable manner to recorded EDS information. For example, if the receiver always uses time information in EDS time-of-day packets to set a clock in the receiver, the clock will be set incorrectly in response to the recorded EDS data because recorded time-of-day information is no longer current. A similar problem exists in regard to EDS packets containing program schedule information because recorded schedule information may be of no value (i.e., the scheduled times have passed).